It is known from German Auslegeschrift No. 1,770,703 that mixtures of incompatible polyols may be used for the production of polyurethanes. In this way, it is possible to obtain end products which have improved properties, such as, for example, higher thermal stability and higher impact strength. According to German Offenlegungsschrift No. 2,309,861, the use of incompatible polyol mixtures is also applicable to the production of foam moldings having compact surfaces.
However, the use of a mixture of incompatible polyols involves numerous disadvantages in material storage and processing. Even brief storage--from a few hours to three days--of thoroughly mixed polyol systems of the type in question causes the mixture to separate into two phases. This means that before processing or packaging, these polyol mixtures must be very intensively mixed or they must be continuously mixed or recirculated to ensure that the ratio between the components remains constant.
Various methods for imparting phase stability to mixtures of this type are known. According to U.S. Pat. No. 3,945,393, the phases may be stabilized by the addition of colloidal silica or a clay modified with an onium compound. German Offenlegungsschrift No. 2,341,294 teaches the use of inert surface-active materials, which have specific surface areas of from 10 to 800 m.sup.2 /g, such as silica agglomerate, a chrysotile asbestos and/or an inorganic material corresponding in its mineral structure to chrysotile asbestos. According to U.S. Pat. No. 4,141,852, mixtures of monomeric polyols having molecular weights below 500 and polyether polyols having molecular weights in the range from 1800 to 7000 are prevented from separating by the use of so-called "graft" polypropylene ether glycols which have molecular weights in the range of from 1500 to 3500. Similarly, U.S. Pat. No. 4,273,884 describes the production of a stable emulsion of a high molecular weight polyol and ethylene glycol or 1,4-butane diol by using an ethylene oxide/propylene oxide copolymer (molecular weight .gtoreq.12,000).
German Auslegeschrift No. 2,759,398 describes mixtures of poly(oxy-propylene/oxyethylene) polyols (OH numbers of from 20 to 60) which have certain oxyethylene contents and ethylene glycol or butane diol. According to U.S. Published patent application No. B471,405, mixtures of high molecular weight polyoxyalkylene polyols having OH-equivalent weights of from 650 to 3000 and, for example, ethylene glycol are prevented from separating by the use of such compounds as 1,2-butylene glycol, di-(1,2-butylene glycol), di-(1,2-propylene glycol) and tri-(1,2-propylene glycol). In addition, it is known that two-phase mixtures of incompatible polyols may be homogenized by the addition of small quantities of emulsifiers, such as, for example, long-chain benzene alkyl sulfonates, etc. None of these prior art solutions is entirely satisfactory, however.
The use of solids as emulsion stabilizers can cause abrasion in mixing units, and indeed the stabilizing effect of these solids generally undergoes a radical deterioration after only a few days. Physiological objections have recently been put forward against asbestos-containing material. The inherent catalytic activity of some of these surface-active substances, particularly where they are modified with onium compounds, must be considered. The use of "graft polyols", as proposed in U.S. Pat. No. 4,141,852, involves a greater expense for these polyols and consequently reduces the economy of the process. Also, the emulsions produced in accordance with U.S. Pat. No. 4,273,884 undergo at least partial phase separation after only 6 to 8 weeks and the patent itself states that only "fairly stable" emulsions are obtained.
Although, as taught in U.S. Published patent application No. B471,405, the use of dipropylene and tripropylene glycol results in homogeneous, storage-stable mixtures from incompatible polyols, it is known that the use of these compounds results in a radical deterioration in the mechanicl properties of polyurethane plastics produced from them, particularly in regard to dimensional stability under heat. Emulsifiers which might be used are capable of gradual migration from the polyol mixture in crystalline form or, for example, of accumulation at the surface or of exuding from the finished polyurethane molding and drastically affecting its performance properties. They may also uncontrollably alter the catalytic balance of the system.
Accordingly, there is still an urgent technical need for polyol formulations which are stable in storage over a sufficiently long period (about 6 months, at least), which are optically clear, do not contain any other solids or liquid additions capable of migrating, otherwise separating, altering or reacting with one another, which have a high mixed OH-number, and which have a high ethylene glycol content.
It has now surprisingly been found that mixtures of polyols and ethylene glycol/propylene oxide reaction products may be produced which are homogeneous, storage-stable, optically clear and have a high content of ethylene glycol, without a simultaneously high content of (basically undesirable, above all higher) propoxylation products of ethylene glycol.